1. Field of the Invention
The present invention relates to a method of forming a ferroelectric film, and more specifically, a method of forming a ferroelectric film with a perovskite structure.
2. Description of the Related Art
Capacitors are used for accumulating charges in a semiconductor memory device. Such capacitors generally include two electrodes and an insulating film therebetween. The insulating film functions as a dielectric material to accumulate charge and is made of a material such as silicon nitride or silicon oxide. Recently, as semiconductor memory devices are more densely integrated, the area occupied by each of the capacitors must be made smaller. As is known, the capacitance of a capacitor is directly proportional to the overlap area of the two electrodes and is inversely proportional to the distance between the two electrodes. The overlap area is substantially the same as the area of the capacitor. Therefore, if the area of the capacitor is made smaller, the capacitance of the capacitor is smaller. As a result, there can arise a problem in that the capacitor can not accumulate enough charge for operating the semiconductor memory device correctly. An approach to solving such a problem is to reduce the distance between the two electrodes or to enlarge the overlap area of the two electrodes by forming the two electrodes with a complex cubic fin structure as is known.
The problem also can be solved by using a insulating film having a high dielectric constant. Metal oxide, such as lead zirconate titanate (hereinafter, referred to as "PZT"), which is denoted as a general formula of ABO.sub.3 and has a perovskite structure is well known for use as a ferroelectric film having a high dielectric constant. The dielectric constant of a silicon oxide film is normally less than ten, but the dielectric constant of the ferroelectric film made of the PZT is more than one thousand. Therefore, in the case where the ferroelectric film is used as the insulating film of capacitors in a semiconductor memory device, the capacitor can have a large capacitance although the electrodes of the capacitor have a small overlap area and a simple structure.
A ferroelectric film can also have a characteristic in that a dielectric polarization of magnets within the ferroelectric film exhibits a hysteresis against an applied electric field. Therefore, after a voltage applied to the ferroelectric film is turned off, the ferroelectric film maintains the same dielectric polarization as the dielectric polarization caused by the applied voltage. A ferroelectric film having such a characteristic can be utilized in a non-volatile memory device, for example.
Hereinafter, a conventional method of forming a ferroelectric film using PZT will be described. A semiconductor substrate having an electrode of a metal such as a platinum formed thereon is heated at a temperature lower than 300 degree Celsius (.degree.C.) during a deposition of the PZT film. While an argon and oxygen gas mixture flows into a reaction chamber, the PZT film is deposited on the electrode under reduced pressure by an RF sputtering method with a PZT sintered target having a molar composite ratio of Ti:Zr:Pb=0.5:0.5:1.15. The thickness of the deposited PZT film is in the range of 200 to 300 nanometers (nm). The PZT film deposited by the RF sputtering method consists of an amorphous film or microcrystals with a pyrochroite structure, and does not have ferroelectric properties such as described above. Then, the substrate is annealed at a temperature in a range of 550.degree. to 700.degree. C. for 30 to 60 minutes. Annealing the PZT film causes a phase transition of the PZT, and the PZT film turns into a ferroelectric film having a perovskite structure.
The above-described method requires annealing the PZT film after the PZT film is deposited. Such annealing causes vaporization of the lead from the PZT film because the lead has the highest vapor pressure of all elements constituting the PZT film. Therefore, the PZT film must contain a larger quantity of lead than ordinarily suitable for a perovskite structure to compensate for vaporization of the lead. Moreover, the vaporization of the lead causes the formation of pinholes in the PZT film. Also, the PZT film tends to crack because of an internal stress or the like derived from annealing the PZT.
On the other hand, in the case where a PZT film is deposited at a high temperature, it is difficult to obtain the PZT film with a perovskite structure. High temperature causes the vaporization of the lead on the metal electrode (such as platinum) and decreases an adhesion coefficient of the lead thereto. As a result, the lead is defective in the PZT film.
In view of the aforementioned shortcomings associated with conventional methods of forming a ferroelectric film, there is a need in the art for a method which produces a ferroelectric film having a uniform composition and perovskite structure.